1. Field of the Invention
The present invention relates to an exposure apparatus and a device fabrication method.
2. Description of the Related Art
A projection exposure apparatus has conventionally been employed to fabricate a semiconductor device using photolithography. The projection exposure apparatus projects and transfers a circuit pattern formed on a reticle (mask) onto, for example, a wafer via a projection optical system using the step & repeat scheme or step & scan scheme.
A minimum dimension (resolution) that a projection exposure apparatus can transfer is proportional to the wavelength of exposure light and is inversely proportional to the numerical aperture (NA) of a projection optical system. Along with the recent demand for micropatterning semiconductor devices, the wavelength of exposure light is shortening and the NA of a projection optical system is increasing. For example, the recent exposure light source is shifting from a conventional super-high pressure mercury lamp (g-line (wavelength: about 265 nm)) to a KrF excimer laser (wavelength: about 248 nm) and ArF excimer laser (wavelength: about 193 nm) each having a wavelength shorter than that of the mercury lamp. An exposure apparatus (scanner) of the step & repeat scheme which uses a KrF excimer laser or ArF excimer laser as an exposure light source has a projection optical system with an NA of 0.7 to 0.95. In recent years, an optical system (immersion optical system) which attains an NA of 1 or more by the so-called immersion which fills the space between a wafer and the final optical lens (final surface) of the projection optical system with a liquid is under development to further increase the NA of a projection optical system.
As the NA of a projection optical system increases, the incident angle of a light beam which enters an optical element near a wafer also increases. The light beam is reflected by the surfaces of the optical element and the wafer (a resist applied on the wafer), so the amount of flare light which reaches inside/outside shot regions increases. For example, if the NA of a projection optical system is less than 0.7, the reflectances of both the final lens (a lens arranged closest to a wafer) of the projection optical system and the surface of a wafer are relatively low, and therefore flare light is negligible. However, if the NA of a projection optical system is 0.7 or more, the reflectances of both the final lens of the projection optical system and the surface of a wafer are relatively high, and therefore the amount of flare light is too large to neglect. The flare light herein generally refers to light which reaches a wafer after multiple reflections at various positions (the peripheral surfaces (edges) of lenses of a projection optical system and the metallic inner surface of a lens barrel).
Flare light in a given shot region generally scatters into its adjacent shot regions. When a plurality of shot regions on a wafer are exposed to light, the amount of flare light which scatters into one shot region is amplified. As a consequence, a pattern is transferred with a size changed from that defined by the design rule by even several tens of nanometers depending upon the shot. This makes it impossible to obtain a desired resolution (exposure performance).
Under these circumstances, Japanese Patent Laid-Open Nos. 2003-17396 and 2006-303340 propose techniques of reducing the adverse influence of flare light on the exposure performance.
Unfortunately, neither prior art techniques can sufficiently reduce the adverse influence of flare light on the exposure performance and avoid a decrease in exposure performance.
For example, Japanese Patent Laid-Open No. 2003-17396 proposes a technique of interposing a stop for shielding flare light between a wafer and the final lens of a projection optical system. However, merely interposing the stop between the wafer and the final lens is insufficient to prevent flare light which passes through the opening of the stop.
Japanese Patent Laid-Open No. 2006-303340 discloses a technique of setting the formation condition of an antireflection coating so that no light is reflected between a wafer and the final lens of a projection optical system (i.e., so that their reflectances become sufficiently low). However, on a resist and antireflection coating, regions in each of which light enters at a relatively large incident angle have relatively high reflectances in principle. This makes it very difficult to form an antireflection coating so that no light is reflected. Furthermore, Japanese Patent Laid-Open No. 2006-303340 does not describe a specific film composition and numerical values.